1. Field of the Invention
The present invention relates to a method for producing a formed body of an α-alumina. More specifically, the present invention relates to a method for producing a high-strength formed body of an α-alumina with a controlled pore volume.
2. Related Art of the Invention
α-alumina formed bodies have been used as carriers for various materials such as chemicals. In these usages, various requests are made on the pore distribution and the pore volume of the carriers. For example, there is a case in that a large pore volume is required to carry a large amount of components therein, or a case in that pore distribution is needed to be controlled to have a predetermined one (see, Japanese Patent Application Laid-Open No. 54-89988, which corresponds to U.S. Pat. No. 4,242,235, or Japanese Patent Publication No. 6-16850, which corresponds to U.S. Pat. Nos. 4,990,481 and 5,100,857). Also, when an alumina formed body is used in a fixed bed facility as a catalyst carrier or an adsorbent, high strength is required to be durable against the collapse thereof during the taking out and filling in of the catalyst. In order to fulfill these requirements, an α-alumina formed body having a large pore volume with high strength has been needed.
Also, in some usages of carrying a catalyst, it is preferred that an alumina formed body has a smaller content of Na2O (hereinafter, referred to as “soda”) in order to reduce segregation of carried metals and the deterioration of the carried components.
Heretofore, as described in Japanese Patent Application Laid-Open No. 52-105587 (which corresponds to U.S. Pat. No. 4,224,302), α-alumina carriers for catalyst are typically produced in one of the following methods (1)-(3).
Method (1): An aluminum hydroxide as a starting material is calcined at a temperature of from 500° C. to 700° C. to obtain an activated alumina powder. The activated alumina powder is mixed with a binder or the like and is then granulated. The resultant grains are calcined and are sintered at a high temperature of from 1400° C. to 1600° C., to obtain an α-alumina carrier with high strength.
Method (2): The starting material as in (1) is calcined at a high temperature of from 1200° C. to 1300° C. to obtain α-alumina powder. The α-alumina powder is mixed with a binder or the like and is then granulated. The resultant grains are calcined and are sintered at a high temperature of from 1400° C. to 1600° C., to obtain an α-alumina carrier with high strength.
Method (3): A gibbsite-phase aluminum hydroxide powder is mixed with a binder or the like and is then granulated. The resultant grains are subjected to hydrothermal treatment to obtain a boehmite. The boehmite is calcined and is then sintered at a high temperature of 1200° C., to obtain an α-alumina carrier with high strength.
In methods (1) and (2), the strength of the obtained α-alumina carriers is insufficient. In method (3), it is difficult to control the pore volume and pore distribution of the α-alumina carrier.
For obtaining an alumina having a reduced soda content, the following methods are known.
Method (4): An aluminum hydroxide with a low soda content, which may be obtained by hydrolysis of aluminum alkoxide, is used as a starting material to obtain an alumina carrier.
Method (5): An alumina formed body with a high soda content is washed with an acid or water, to obtain an alumina carrier.
In methods (4) and (5), an alumina carrier (or formed body) with a low soda content is obtained. However, method (4) has problems such that the production method is complicate and costs high. Method (5) has problems such that the alumina itself is eluted by the acid treatment, causing reduction in strength and that the method requires a cost for wasted water treatment and thus is not necessarily economical.
For controlling pore distribution of an alumina, a method including addition of an organic substance is generally known (see, Japanese Patent Application Laid-Open No. 4-160078). However, this method has problems such that the cost is high because an organic substance is necessary and that the strength of the obtained alumina tends to significantly decrease with increase of the pore volume.